The invention also relates to a device for determining a control parameter for a fuel injector of an internal combustion engine, with a measuring unit with which at least one fuel pressure value that is present at the fuel injector is able to be recorded and with an evaluation unit with which a control parameter for the fuel injector is able to be determined in consideration of the recorded fuel pressure value.
Increasing numbers of legal specifications regarding exhaust emission characteristics and specifications from the vehicle manufacturers require ever more precise feed-forward control of the fuel metering in internal combustion engines. In most combustion methods, it is necessary to calculate and feed forward the air-fuel ratio or the fuel mass supplied to the combustion chamber per combustion cycle as precisely as possible. In this way, for example, the homogenous combustion that is usual in petrol engines requires an air-fuel ratio that is as accurate as possible for catalytic-converter management on the one hand and, on the other hand, for temperature management in the combustion chamber and in the components carrying the exhaust gases. More complex combustion methods, such as HCCI (homogenous charge compression ignition) or CAI (compressed auto ignition) set combustion parameters, such as the ignition point and rate of combustion, with the help of accurate fuel mass deliveries.
Fuel injectors are used to apportion the required fuel mass into the cylinders of the internal combustion engine. In the present context, the term fuel injector shall be understood to include all types of control elements that are used to apportion the required fuel mass into the cylinders (injectors, fuel injection valves, etc.). Usually, a fuel injector is an electrically actuated control element. As well as single injections, multiple injections, i.e. more than one injection per combustion cycle, are also used in all combustion methods, in particular in engines that are operated with direct fuel injection.
The conversion of the fuel mass to be apportioned into, in particular, an electrical actuation signal of the fuel injector is dependent on the fuel pressure that is present at the fuel injector. This fuel pressure is therefore measured, filtered if necessary and fed into a model, which calculates a control parameter, for example the injection time, corresponding to the desired injection quantity as a function of the recorded fuel pressure. Fuel is then injected by the fuel injector for the duration of this injection time.
Conventional engine management systems have several software levels, real-time software (also called IO software) and application software. For example, the injection time and start or stop time of the injections are continually calculated by the application software and passed on to the IO software. The real-time software receives the data in fixed time slots and/or event slots from the application software and executes it by implementing the specifications autonomously.
It is thereby conceivable for changes to be made to the injection time, for example, even during a current injection. A second injection may even be activated. In this way, it is possible to implement changes in the fuel mass that is to be injected particularly quickly.
A problem is presented by the dependency of the calculation of control parameters, such as the injection time calculation, on the injection pressure, that is the fuel pressure that is present at the fuel injector. In this way, too much or too little fuel may be injected, in particular if the present fuel pressure changes between the time that the injection time is calculated and the time of the injection, depending on whether the fuel pressure increases or decreases. Injections of the incorrect quantity of this kind result in an unfavorable operating performance of the internal combustion engine, for example increased exhaust emission values.
A corresponding problem occurs if, following a completed injection, the injection quantity or injection time of the first injection is checked, such that an incorrectly injected quantity in the first injection can be corrected with a second injection if necessary. If the present fuel pressure has changed between the first injection and the time of the check calculation for the associated injection quantity or time, the check calculation may return the result that an incorrect quantity was injected in the first injection, even though this was not actually the case. A subsequent corrective injection would not therefore result in an improvement, but in a deterioration of the injection result.
Attempts have been made to compensate for lambda deviations generated by incorrect injection with the fuel mass during changes in the operating point by means of transitional characteristic maps. Attempts have also been made to configure the lambda controllers in such a way that they can react to deviations due to incorrectly injected quantities. The exhaust gas cleaning components and other components of motor vehicles have also been dimensioned or arranged in such a way that they can compensate for any deviations in the injected fuel quantity and withstand thermal interference caused by lambda fluctuations. Finally, attempts have been made to make practical allowances in the lambda setpoint in order to keep the internal combustion engine in a safe lambda range for the components at all times. However, a satisfactory solution for the problem discussed above does not currently exist.